The most abundant group of fats are triglycerides--esters of fatty acids with glycerol (1,2,3-propanetriol). Natural fats have a broad range of functionalities and are handled in different ways by the human digestive process.
Early studies reported that triglyceride fats having high melting points were less digestible (Deuel, H. J., The Lipids, vol. II, Interscience Publishers, 1955, pages 218 to 220). Later investigators questioned the relationship between digestibility and melting points, and scrutinized instead the chain lengths and degree of unsaturation of fatty acid substituents; straight chain, saturated fatty acids having 4 up to 10 carbon atoms were completely digested by rats, those having 10 to 18 carbons progressively less digested, and those having 18 or higher only slightly absorbed, while monounsaturated acids were about the same as saturated acids having 6 carbons (Carroll, K. K., J. Nutr. 64: 399-410 (1957) at 408).
In other triglyceride metabolic studies in man only limited areas of predictability could be found. In one study a coconut oil fraction containing predominantly saturated, long chain triglycerides bearing 89% stearic (C.sub.18) and 11% palmitic (C.sub.16) acid residues were absorbed 31%, compared to 98% for corn oil (Hashim, S. A., and Babayan, V. K., Am. J. Clin. Nutr. 31:S273-276 (1978)). However, it was found that increasing the stearic acid content of dietary fat did not per se decrease absorbability; rather, absorbability could be decreased by increasing the amount of tristearin present (i.e., triglycerides having three stearic residues; see Mattson, F. H., J. Nutr. 69: 338-342 (1959)). To this observation were added the findings that, in the presence or absence of dietary calcium and magnesium, stoatic acid was well absorbed by rats when esterified on the 2-position of triglycerides having oleic acid at the 1- and 3-positions, but absorption decreased when a second stearic was added to the 1-position (Mattson, F., et al., J. Nutr. 109: 1682-1687 (1979), Table 3, page 1685). Stearic acid in the 1-position was well absorbed from triglycerides having oleic in the 2- and 3-positions in the absence, but not in the presence, of dietary calcium and magnesium (ibid.). When stearic was in both the 1- and 3-positions, absorption decreased with or without dietary calcium and magnesium, but the effect was more pronounced when calcium and magnesium were sufficient (ibid.).
The digestibility of palmitic acid has also been studied. Palmitic acid was better absorbed by rats when situated at the 2-positions of triglycerides than at the 1- or 3- positions in naturally occurring fats commonly fed to infants, and total fat absorption was adversely influenced by increasing the palmitic and stearic acid content in the 1- and 3-positions (Tomerelli, et al., J. Nutr. 95: 583-590 (1968)).
While triglycerides high in stearic acid are less well utilized than others, they also tend to be high melting. Tristearin is a solid at room temperature; the alpha form is a white powder that melts at 55.degree. C., which, on solidification, reverts to the beta form that melts again at 72.degree. C. The melting points of 1,3-distearin with short or medium chain fatty acids at the 2-position are high (Lovegren, N. V., and Gray, M. S., J. Amer. Coil Chem. Soc. 55: 310-316 (1978)). Symmetrical disaturated triglycerides of stearic acid and/or palmitic, often with oleic at the 2-position, melt fairly uniformly near body temperature, and this property is of advantage for cocoa butter and hard butter substitutes (see, for example, U.S. Pat. No. 4,364,868 to Hargreaves, U.S. Pat. No. 4,839,192 to Sagi, et al., and U.S. Pat. No. 4,873,109 to Tanaka, et al.), and for hardstocks for margarines and shortenings (see, for example, U.S. Pat. No. 4,390,561 to Blair, et al., U.S. Pat. No. 4,447,462 to Tafuri and Tao, U.S. Pat. No. 4,486,457 to Schijf, et al., U.S. Pat. No. 4,865,866 to Moore, and U.S. Pat. No. 4,883,684 to Yang). Because of their functionality, high melting, high stearic fats have limited applications in food compositions requiring more plastic or liquid triglycerides.
Fats have been prepared by substituting acetic acid for a portion of the fatty acids occurring in ordinary fats or oils, thus producing triglycerides bearing short acetyl and long substituents. For saturated fats high in stearic acid, the substitution of acetyl groups for a portion of the stearyl groups lowers the melting point. These aceto-glycerides were investigated during the 1950's and found to be digestible. Feeding studies indicated that the nutritive value of mono- and diacetin fats were essentially the same to animals as those fed the corresponding conventional triglycerides (Mattson, F. H., et al., J Nutr. 59: 277-285 (1956), although acetooleins were more digestible than acetostearins (Ambrose, A. M., and Robbins, D. J., J. Nutr. 58: 113-124 (1956) and animals grew poorly when fed acetostearin as the sole dietary fat (Coleman, R. D., et al., J. Amer. Oil Chem. Soc. 40:737-742 (1963)).
While lower melting than tristearin, acetostearins still have high melting points, limiting applications in food products requiring plastic or liquid fats. In fact, though melting points of compounds structurally related generally decrease with decreasing molecular weight (and mono- and distearins having medium to long saturated substituents follow this rule), the melting points of triglycerides in the C.sub.18 C.sub.n C.sub.18 and C.sub.n C.sub.n C.sub.18 series, where n=2 to 6, anomalously show the high molecular weight C.sub.6 (caproic acid) mono- and distearin derivatives to have the lowest melting points and the low molecular weight C.sub.2 (acetic acid) mono- and distearin derivatives to have the highest (Jackson, F. L., et al., J. Amer. Chem. Soc. 73: 4280-4284 (1951) and Jackson, F. L., and Lutton, E. S., J. Amer. Chem. Soc. 74: 4827-4829 (1952); see also the data in Example 38). Plastic fats containing acetostearins suggested for use as shortenings and the like were formulated to contain significant levels of unsaturated fats and typically employed significant levels of fatty acids which would yield high saponification numbers or were liquid at room temperature (U.S. Pat. No. 2,6714,937 to Baur and Lange (1952) and Baur, F. J., J. Amer. Oil Chem. Soc. 31: 147-151 (1954)).
Acetostearins are waxy fats having sharp melting points. In contrast to fats bearing medium and/or long substituents, acetostearins also exhibit unusual polymorphism (ibid., and Feuge, R. O., Food Technology 9: 14-318 (1955)). Because of their melting and crystal properties, the fats have been suggested as useful for coating food products such as meat, fish, cheese, and candy (U.S. Pat. No. 2,615,159 to Jackson and U.S. Pat. No. 2,615,160 to Baur). Compositions of this nature are often referred to as "hot melts" and may contain antibiotics (U.S. Pat. No. 3,192,057 to Hines and Shirk) or polymeric materials (U.S. Pat. No. 3,388,085 to Levkoff and Phillips) to prolong the life of the coating.
The short chain fatty acids, acetic, propionic, and butyric acid, also called, as a group, volatile fatty acids, occur in the large intestine of all mammalian species so far studied (Cummings, J. H., Gut 22: 763-779 (1981)). Except for a small percentage of butyric acid in milk fat (i.e., about 3.5 to 4%), volatile fatty acids rarely occur in nature esterified to glycerol in fats, but are, instead, generally free by-products of fermentation in the gut. Physically, short chain fatty acids "are not at all `fatlike` in character; in fact they are hydrophilic substances with complete miscibility with water" (Bailey's Industrial Oil and Fat Products, 4th. ed., J. Wiley, New York, 1979, volume 1, pages 16 to 17).
Early reports investigating the metabolism of short acids and triglycerides bearing short chain residues showed no regular relationship between nutritional value and the number of carbon atoms in the fat (Ozaki, J., Biochem. Z. 177: 156-167 (1926) at 163). For example, when fed to rats at levels of 5% and 10% of the diet, triacetin and tributyrin were nutritious, yielding weight gains in the top 20 to 25% of the fats tested, whereas tripropionin and triisovalerin were toxic (ibid.). In 1929, Eckstein reported that rats fed trioloin and sodium butyrate grew at the same rate (J. Biol. Chem. 81: 163-628 (1929) at 622).
In 1935, L. E. Holt, et al., observed that infants fed milk enriched with tributyrin retained more fat per day (90.1 to 90.2%) than those in a butterfat control group (88.9%); the study concluded that absorption was favored by fatty acids with relatively short chains (J. Ped. 6: 427-480 (1935), Table VIII, page 445, and Conclusions, number 4, page 477). Similar results were obtained with triacetin, with absorption of tributyrin and triacetin reportedly superior to that of corn oil, although corn oil yielded higher calories (Snyderman, S. E., et al., Arch. Dis. Childhood 30: 83-84 (1955)). Substitution of triacetin, tripropionin, or tributyrin for half the glucose and starch in a rat diet did not significantly affect the digestible, metabolizable or net energy measurements, but lower body weight gains were observed in animals fed tributyrin in two experiments and triacetin in one experiment (McAtee, J. W., et al., Life Sci. 7: 769-775 (1968)).
In in vitro digestibility studies, tributyrin is readily cleaved by pancreatic lipase. Data measuring lipolysis as a function of chain length show tributyrin much more rapidly hydrolyzed than other substrates (see Sobotka, H., and Glick, D., J. Biol. Chem. 105: 199-219 (1934), comparing triglycerides bearing three identical C.sub.4 to C.sub.18 acyl groups, and Desnuelle, P., and Savary, P., J. Lipid Res. 4: 369-384 (1963), comparing triglycerides bearing three identical C.sub.2 to C.sub.18 acyl groups), although some reports rank tripropionin slightly better (Weinstein, S. S., and Wynne, A. M., J. Biol. Chem. 112: 641-649 (1936), comparing triglycerides bearing three identical C.sub.2 to C.sub.6 acyl groups, and Wills, E. D., in Desnuelle, P., ed., The Enzymes of Lipid Metabolism, Pergamon Press, N.Y., 1961, pages 13 to 19, comparing triglycerides bearing three identical C.sub.2 to C.sub.18 acyl groups). In fact, because tributyrin is such a good substrate and because the triglyceride is sufficiently water-soluble to allow enzymatic measurements in a homogeneous solution, it is often selected as a lipase substrate standard (Ravin, H. A., and Seligman, A. M., Arch. Biochem. Biophys. 42: 337-354 (1953) at 353).
Other lipase preparations readily cleave short chain triglycerides. Tributyrin was found to be hydrolyzed with the greatest initial velocity by human milk lipase, while pig liver lipase hydrolyzed tripropionin and tributyrin with an equal initial velocity much greater than any other in a study comparing C.sub.2 to C.sub.18 triglycerides (Schonheyder, F., and Volqvartz, K., Enzymologia 11: 178-185 (1943)). Tributyrin was hydrolyzed more readily than C.sub.6 to C.sub.18 triglycerides by human milk bile salt-activated lipase (Wang, C. S., et al., J. Biol. Chem. 258: 9197-9202 (1983)). A liver lipase hydrolyzed trivalerin the fastest, with tributyrin the second fastest (Sobotka and Glick, cited above).
In contrast to triglycerides bearing long chain (.about.C.sub.16 to C.sub.24) fatty acids and those bearing short chain fatty acids, medium chain triglycerides, generally obtained from kernel oils or lauric fats and encompassing those substituted with C.sub.6 to C.sub.12, predominantly C.sub.8 to C.sub.10, fatty acids, have been of particular interest because they are more rapidly absorbed and metabolized, via a different catabolic route than those bearing long chain fatty acids (see a recent review by Babayan, V. K., in Beare-Rogers, J., ed., Dietary Fat Requirements in Health and Development, A.O.C.S. 1988, chapter 5, pages 73 to 86). Hence, medium chain triglycerides have been employed in premature infant formulas and in the treatment of several malabsorption syndromes (ibid.). Feeding studies by H. Kaunitz, et al., demonstrated the usefulness of medium chain triglycerides in weight maintenance and obesity control in rats (J. Amer. Oil Chem. Soc. 35: 10-13 (1957)).
Several research groups have exploited the physical and nutritional properties of medium chain fatty acids by suggesting that triglycerides having stearic and/or behenic acid in combination with medium chain substituents be used as low calorie fats (Eur. Pat. Ap. Pub. No. 322,027, corresponding to U.S. application Ser. No. 132,400, to Seiden, who defined medium chain substituents as comprising C.sub.6 to C.sub.10 residues, and Jap. Pat. Pub. No. 2-158,695 to Yoshida, et al., who defined medium chain substituents as comprising C.sub.4 to C.sub.12 residues. The latter publication, however, exemplified only trace amounts of C.sub.4 fatty acids, and suggested incorporating 0 to 1 long chain, unsaturated residues as well.) Low calorie triglyceride mixtures having stearic acid at the 1-position and medium and unsaturated residues in the other positions have also been suggested (U.S. Pat. No. 4,832,975 to Yang).
The polymorphism of triglycerides bearing medium and long moieties generally resemble fats bearing long moieties in that they tend to have a stable beta crystal structure. This contributes to graininess of fat mixtures containing them, and, in chocolate compositions, to the appearance of bloom. The preparation of smooth blends require careful substituent selection and/or tempering. It would be desirable to have low calorie fat mixtures free of this disadvantage. It would also be desirable to have a fat which was a true triglyceride but which delivered a minimum of calories and exhibited functionalities which permitted use in a wide variety of products.